1. Field of the Invention
The present invention relates to transmission methods in one or more wireless base stations, wireless base stations, and wireless communication methods.
2. Description of the Related Art
The present cellular mobile communication system divides a service area into a plurality of cells, where wireless base stations serving there are placed. In the interest of communication capacity, the frequency band that can be used in each cell of the cellular system should be as broad as possible. However, if a plurality of cells (particularly adjacent cells) use the same frequency band, an interference can occur. To implement communication of good quality, the same frequency band should not be used among adjacent cells at least.
A fractional frequency reuse (FFR) technology is also known. FFR provides a time zone referred to as a reuse one (R1) region and a time zone referred to as a reuse three (R3) region, in a radio frame. In the R1 region, a broad frequency band (system band) is used. In the R3 region, a part of the frequency band used in the R1 region is used, the part being allocated to each cell, avoiding overlaps among adjacent cells. The wireless base station measures the quality of communication with each mobile station in the cell and communicates with the mobile station in the R1 region if the quality of communication is high or in the R3 region if the quality of communication is low (for example, refer to “Mobile WiMAX—Part I: A Technical Overview and Performance Evaluation”, WiMAX Forum, 2006-08).
Another method places an additional wireless base station at the center of each cell and forms a small-radius concentric cell (internal zone) inside the cell. In this method, the cells (external zones) use different frequency bands that do not overlap among adjacent cells, and all the internal zones use a common frequency band. When a mobile station is placed outside the internal zone, the mobile station communicates with the wireless base station which manages the cell. When a mobile station is placed in the internal zone, the mobile station communicates with the base station which manages the internal zone (refer to Japanese Unexamined Patent Publication No. 05-63634, for example).
In order to increase the speed and capacity of wireless communication, the radius of each cell has been reduced in recent years to decrease the number of mobile stations to be served in the single cell, so that more wireless resources can be allocated to each mobile station. In one system, a single base station does not form a communication zone, but a single wireless base station forms a plurality of communication zones (sectors) by emitting directional beams (of 120 degrees, for instance) instead of an omnidirectional beam (of 360 degrees).
A system using a sector structure manages use of the band for each sector. Therefore, interference with another sector in the same cell should be considered in addition to interference with an adjacent cell.
The method described in “Mobile WiMAX—Part I: A Technical Overview and Performance Evaluation”, WiMAX Forum, 2006-08, however, has poor radio transmission efficiency. Because the quality of communication often drops in the vicinities of sector boundaries (boundaries with other cells and boundaries with other sectors), the most of mobile stations in the vicinities of the sector boundaries turn out to communicate in the R3 region. Therefore mobile stations which communicate in the R1 region are the mobile stations near the centers of the sectors. Accordingly, the mobile stations in the vicinities of the sector boundaries do not require radio waves in the R1 region.
With the method described in Japanese Unexamined Patent Publication No. 05-63634, radio waves in the internal zone do not reach the vicinity of the cell boundary, which is preferred in terms of radio transmission efficiency. However, it is difficult to have coordination between the external zone and the internal zone, and the communication control efficiency is low. One reason is that different wireless base stations manage the external zones and the internal zones, and independent control channels are provided. A mobile station in the vicinity of the boundary between the external zone and the internal zone must select and communicate with either the wireless base station which manages the external zone or the wireless base station which manages the internal zone. This makes it difficult to allocate a frequency band in a flexible manner in accordance with the actual quality of communication.